1. Field of the Invention
The present invention concerns a novel method for preparing polymers (preferably (meth)acrylate polymers and copolymers) having a narrow molecular weight distribution by metal-free anionic polymerization in the presence of a phosphonium cation containing at least one annellated aromatic ring as substituent on the phosphorous atom.
2. Discussion of the Background
In the last decade, great interest has emerged concerning the polymerization of (meth)acrylates, particularly methyl methacrylate (MMA), under ambient conditions. A number of new polymerization systems have been reported, and a recent review covers most of the literature on the polymerization of MMA at ambient temperatures (T. P. Davis, D. M. T. Haddelton, S. N. Richards, J. M. S.-Rev. Macromol. Chem Phys, 1994, C34, 243).
For example, a process known as Group Transfer Polymerization (GTP), introduced in the early 1980's, produced poly(methyl methacrylate) (PMMA) and allowed control of molecular weight distribution (MWD), molecular weight and molecular architecture at ambient temperatures (Webster et al. J. Am Chem. Soc. 1983, 105, 5706).
Reetz et al (Angew. Chem. Int. Ed. Engl. 1988, 27, 1371; Polymer Preprints (Am. Chem. Soc., Div. Polym. Chem.) 1991, 32, 296) have postulated that, in the anionic polymerization of n-butyl acrylate at ambient temperatures using a tetrabutylammonium countercation, the intramolecular Claisen type termination reaction is decreased because the electrostatic attraction between the alkoxide and the bulky n-Bu.sub.4 N.sup.+ cation is weak, thereby thermodynamically and kinetically disfavoring the formation of the termination by-product tetrabutylammonium alkoxide. However, anionic polymerizations with ammonium countercations also suffer from some drawbacks, such as low yield, which may be the result of a Hoffmann elimination of a .beta.-hydrogen from the ammonium cation at a rate competitive with polymerization.
A distinct improvement on the methods described is constituted by the anionic polymerization in the presence of phosphonium counterions (WO 96/28487, U.S. patent application Ser. No. 08/609,732). According to WO 96/28487 the polymerization is carried out at temperatures of up to 40.degree. C.
The polymerization of acrylic acid derivatives at temperatures of more than 20.degree. C. and particularly between 40 and 120.degree. C. in the presence of an initiator corresponding to or containing the structure ##STR1## where E may be CR.sup.IV R.sup.V and where C together with R.sup.IV and R.sup.V may form an unsaturated isocyclic ring was disclosed in DE-A 36 32 361 and DE-A 37 00 195. Since a phosphonium species is found to be covalently linked to the polymer chain formed a mechanism according to Klippert and Ringsdorf (Makromol. Chem. 1972, 53, 289-306) where the ylide carbon atom attacks the acrylic or methacrylic monomer can be postulated. The initiator (V) can also be used in its protonated form. As shown in the examples the best results are obtained when the initiator is employed as Lewis acid adduct. Merely very poor yields, incomplete monomer conversion and polymers having a broad molecular weight distribution are obtained when the reaction is performed in the presence of the neutral catalyst. In the case of the protonated species the acidic proton may cause chain termination. Therefore, the scope of the polymerization using (V) is limited.
Another entry to metal-free anionic polymerization of acrylates and methacrylates disclosed by Seebach and Pietzonka (Angew. Chem. Int. Ed. Engl. 1993, 32, 716) makes use of the P4-phosphazene base introduced by Schwesinger and Schlemper (Angew. Chem. 1987, 99, 1212) which forms a very soft and bulky cation. The initiating system can for example be prepared by treating ethyl acetate with the P4-phosphazene base. Polymerization of for example methylmethacrylate is accomplished at 60.degree. C. furnishing predominantly a syndiotatic polymer material. However, the P4-phosphazene base is not readily available and therefore not favorable for large scale production. In addition, the formation of block copolymers has not yet been disclosed.
Thus, a need exists for a method for producing poly(meth)acrylate polymers and copolymers having a narrow molecular weight distribution, which provides effective control of (co)polymer molecular weight, of molecular weight distribution and of (co)polymer stereoregularity in good yields and at technically favorable temperatures, also allowing for large scale production.